Add load_benchmark_data management command for performance testing

[awais786]

This pull request introduces support for generating and loading benchmark data into the Bakerydemo project by implements GitHub issue #570 . Adds load_benchmark_data management command for performance testing.

# Run with defaults
python manage.py load_benchmark_data

# Create 100K pages for scale testing
python manage.py load_benchmark_data --blog-pages-count=100000

# Full benchmark configuration

Instead many params just define low, medium and high for simplicity.

python manage.py load_benchmark_data --scale low

[awais786]

@thibaudcolas This is basic draft PR. I am planning to add small prs in incremental way. Just want to confirm I am on right track ?

[awais786]

@thibaudcolas Kindly review.